Method and apparatus for enhancing image contrast using intensity filtration

ABSTRACT

An intensity filter for deep UV lithography enhances contrast and also therefore increases the resolution of patterned images by passing only intensities that fall within a specific minimum threshold value, resulting in a more exact aerial image replicating the mask image. This device is a different approach to contrast enhancement that is distinguished from previous methods by eliminating the need for an extra layer of contrast enhancement on top of the resist, thereby reducing the number of processing steps in semiconductor fabrication.

This is a divisional of application Ser. No. 09/557,946, filed Apr. 24,2000 now U.S. Pat. No. 6,549,322, which is a continuation of applicationSer. No. 09/106,720, filed Jun. 29, 1998, now abandoned.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates generally to the field of semiconductorfabrication and in particular to photolithographic processes used in themanufacture of semiconductor devices. Still more particular, the presentinvention relates to a method and apparatus for reducing degradation inthe contrast of resist images produced from a mask image.

2. Description of the Related Art

Microcircuit fabrication requires that precisely controlled quantitiesof impurities be introduced into very small regions of a substrate.These regions are subsequently interconnected to, created components andvery large scale integration (VLSI) circuits. The patterns that definethese regions are created by lithographic processes. Typically,photoresist materials are spun onto a wafer substrate. Then, thephotoresist is selectively exposed to radiation, such as ultravioletlight, electrons or x-rays. An exposure tool and a mask are used tocause the desired exposure of the photoresist. The patterns in theresist are formed when the wafer under goes a development step. Theareas of photoresist remaining after development of the photoresistprotect the covered regions of the substrate during introduction ofimpurities or during etching of exposed regions of the substrate.

In the art of deep ultraviolet microlithography, much demand astechnology develops is placed upon increased resolution, tighterplacement of features in proximity to one another, and smaller butwell-defined features. In order to continue meet increasing demands,many methods and devices have been developed and tested. One suchconcept is the control of images exposed onto a resist layer byenhancing the contrast of the said images.

In the process of patterning an image on a layer of resist on a wafer,light passes through openings on a mask, then through a lens chamber,and finally onto the wafer that is coated with resist. A “mask” is apattern tool, which contains patterns that can be transferred to anentire wafer or to another mask in one exposure. At the resist levelportions of the resist that coincide with the image pattern are exposedand then developed to reproduce the same original image in a greatlyreduced form. When light passes though the mask sub harmonics areintroduced into the intensity distribution of the aerial image. Thesesub harmonics cause the aerial image formed at the resist layer to be aninexact replica of the mask image. The resist image formed from the rawaerial image tends to have degraded contrast using presently availableprocesses and equipment due to coinciding harmonics that are transmittedin addition to the desired zeroeth order element. Therefore, the samesub harmonics also limit the maximum resolution allowed in the finalimage. In order to reduce the distortions, previous methods improved thecontrast by relying on the use of an extra layer that is formulatedspecifically for contrast enhancement coated on top of the resist layer.

However, such methods lengthen the processing of semiconductors byrequiring additional process steps to place a coat of a contrastenhancement layer on top of the resist layer.

Therefore it would be advantageous to have a system and a method forenhancing contrast with minimal additional process steps.

SUMMARY OF THE INVENTION

The present invention provides an intensity filter for deep ultravioletlithography for enhancing contrast. The intensity filter filters lighthaving various intensities. The intensity filter includes a firstmaterial and a second material in which these two materials interface insuch a way that only specific intensities are passed through. The firstmaterial is non-linear in nature and has a refractive index that changesat high intensities but has a constant refractive index substantiallyequivalent to the second material at a selected intensity. The secondmaterial has a constant refractive index irregardless of varying levelsof intensity, at intensities lower than a specific minimum threshold.The filter also may include a coating that will phase shift the exitingfiltered light 180-degrees.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features believed characteristic of the invention are setforth in the appended claims. The invention itself however, as well as apreferred mode of use, further objects and advantages thereof, will bestbe understood by reference to the following detailed description of anillustrative embodiment when read in conjunction with the accompanyingdrawings, wherein:

FIG. 1 is a schematic illustrating the configuration of the inventionitself and its relative positioning within a simplified exposure systemin deep ultraviolet lithography;

FIG. 2 is a schematic demonstrating the effects of the invention onvarious degreed orders of light at different stages of the process flowthrough and resultant of the present invention;

FIG. 3 is a defraction pattern graph;

FIG. 4 is a relative intensity distribution graph;

FIG. 5 is a relative intensity distribution graph depicting the resultof filtering the light as it passes through the phase shifted intensityfilter cell in accordance with a preferred embodiment of the presentinvention; and

FIG. 6 is a relative intensity distribution graph of light left from thefiltered light is depicted in accordance with a preferred embodiment ofthe present invention.

DETAILED DESCRIPTION

The present invention provides a method and apparatus for passing onlyintensities that fall below a specific minimum threshold. The minimumthreshold value can be set equivalent to the intensity of the zeroethorder harmonic. This value can be derived for the smallest feature sizeon the mask. By so doing, the zeroeth order harmonics for all the otherfeatures will be greater than the zeroeth order element for the minimumfeature. Thereby the resulting image will be a “filtered” version of theaerial image consisting only of side lobes. Rather than compensating forthe additional harmonics at the resist level, the present inventioneliminates sub harmonics before they imprint on the resist itself.Therefore, the intensity filter system provided for by the presentinvention acts as a low pass filter.

The filter used in the present invention includes two properties. Oneproperty of the filter is that it should not pass light of an intensityabove a certain critical value. The other property of the filterrequires that it does pass light of any intensity below that specifiedvalue. The present invention provides for constructing an intensityfilter cell comprised of at least two materials. One desired materialhas a linear or constant refractive index. The other material has thequality of a non-linear or an intensity dependent refractive index.These two materials interact together in such a way that if placedcorrectly in the form of an enclosed cell, a very high contrast imagewill form in the resist after being developed. The two materials in thedepicted example are selected in such a way that the refractive indicesof the materials are equivalent at one or more selected intensities,such as, low intensities, and the elements are also to be essentiallytransmissive. In the depicted examples, low intensities are defined asintensities that are less than the intensity of the first harmonic ofthe smallest feature of the mask. A low intensity typically about 10 mJ.In the depicted examples, high intensities are defined as intensitiesequal to or greater than the intensity of the first harmonic of thesmallest feature of the mask. High intensities are typically greaterthan 10 mJ. At high intensities, determined by the zeroeth orderharmonic of the minimum feature, the refractive index of the non-linearmaterial, will chance substantially enough to cause the combinedelements to have a mismatched refractive index. This mismatch in therefractive indexes between the two materials leads to poor transmissionof light or even complete internal refraction. This event results in theblocking out of the high intensity components of the aerial image.

In addition to employing refractive index compatibility to filter outunwanted sub harmonics, the filter of the present invention also mayemploy a coating to enhance contrast in the depicted example, thecoating has a thickness such that a 180-degree phase shift in the lightis produced. In the depicted example, the thickness of the coating forthe intensity filter cell can be determined by the equation:t=λ/4,where λ is equal to the wavelength of the incident light. The filteredand phased shifted light is then combined with the original light havingan intensity distribution present prior to filtering using the intensityfilter. The result of this union of the filtered and phase shifted lightand the original light has an aerial image comprising of only theprinciple harmonic of the image. When this image is made incident on thephoto resist layer, the contrast is markedly improved, thereby improvingthe resolution as well.

With reference to the figures and in particular to FIG. 1, a diagram ofa photolithography apparatus is depicted in accordance with a preferredembodiment of the present invention. Photolighography system 100receives light 102 at a specifically designed mask 104 from anillumination source (not shown). Mask 104 is used to describe a patternto be exposed on a photo resist layer. Mask 104 is a photomask. Light102 upon passing through the various openings in mask 104 is thendiffracted resulting in sub harmonics being thereby introduced into theintensity distribution of the aerial image. The resultant light thenenters the photolithography lens 106, which reduces the image size tothe appropriate ratio, then passes into the beam splitter 108. Beamsplitter 108 causes the light 102 to divide into two light beams 109 and111. Light beam 109 travels along a path created by a light guidingapparatus, which in the depicted example includes mirror reflector 110and mirror reflector 112. Light beam 111 travels through intensityfilter cell 114. The light beam 111 is rejoined with light beam 109 asan unfiltered version of the original light with the additional subharmonics.

Light beam 111 is filtered as it passes through intensity filter cell114, which includes material 116, material 118, anti-reflective layer120, anti-reflective layer 122, phase shifter 124 and high intensityabsorber 125. Material 116 is a material that has a refractive indexthat is linear, or constant. Material 118 has a refractive index that isnon-linear, or intensity dependent, nature. A linear material is amaterial that has a constant index of refraction in which the index ofrefraction is independent incident intensity which is described asfollows:n=n ₀wherein n is the index of refraction and n₀ is a constant. This constantis typically selected to correspond to the refractive index of the lenselement, such as photolithography 106 in FIG. 1. In the depictedexample, a non-linear material is a material that has a constant ofindex of refraction at a low intensity and a index of refractiondependent on the intensity of incident light at a low intensity. Theindex of refraction of a non-linear material may be described asfollows:n=n ₀ ,E<Ean=n ₀ ±n ₀ *E/Ea, E>Eawhere E is the intensity of incident light and Ea is the intensity atwhich the material switches from a constant index of refraction to anintensity dependent refractive index of refraction. An example of anon-linear material is MBBA: N-(p-methoxylbenzylidene)-p-butylaniline.

A large class of well-known organic and non-organic non-linear opticalmaterials exists, wherein these materials each have a refractive indexthat changes with the incident intensity. Each of these materialspossesses one constant index of refraction when (1) the intensities fallbelow a specific critical intensity level, and (2) has a differentrefractive index that is variable when the intensities surpass thatspecific level of intensity. Material 116 and material 118 are chosensuch that the pertinent refractive index of each at specific levels ofintensity reacts in a predetermined manner. Material 116 and material118 are selected such that at low intensities the refractive index ofeach are equivalent to the other index of refraction and therefore aretransmissive in accordance with a preferred embodiment of the presentinvention. However, the refractive index of non-linear material 118 willchange at high intensities and results in non-identical refractiveindices, which results in poor transmission of light, or ideallycomplete internal refraction. In the depicted example, the highintensity levels are established by the zeroeth order harmonic of theminimum feature of the pattern to be made incident on the resist layer.Examples of suitable linear materials include quartz or fused silica.Examples of some materials that may be used for non-linear opticalmaterial 118 are as follows:

MBBA: N-(p-methoxylbenzylidene)-p-butylaniline

In the depicted example, positioning of material 116 and material 118within intensity filter cell 114 provides one possible configuration.High intensity absorber 125 may be formed using a glass plate coatedwith an absorbing film, such as silicon. High intensity absorber 125 inthe depicted example is positioned such that it is in contact with oneedge of material 118 as shown in FIG. 1. If the order of the two opticalmaterials are reversed, i.e., material 118 and material 116 areswitched, the absorber would be moved to the opposite side of intensityfilter cell 114, such that it is still in contact with material 118. Inthe depicted example, the intensity filter cell will have a dimension atleast equal to a reduced image of the mask after passing through thelens. These dimensions are typically about 25 mm by 25 mm. The filterestablishes a filtered variation on the original light beam. Material116 and material 118 are enclosed between anti reflective layers 120 and122. The light exiting intensity filter 114 then passes through thephase shifter 124, wherein the phase of the light is shifted180-degrees. The necessary thickness of the coating comprising phaseshifter 124 is determined by the equationt=λ/4,where λ is equal to the wavelength of the incident light.

A number of different types of phase shifting materials may be used forthe coating. For example, chrome and calcium flouride may be used as acoating. Phase shifter 124 is employed to reverse (or to phase shift by180 degrees) the intensity distribution coming out of a filter. Whencombined with an unshifted distribution of light in light beam 109, allof the subharmonics except the zeroeth order are removed, as illustratedin FIG. 6 below. When the light exits the intensity filter cell, thelight has specific intensities filtered out and is phase shifted. Theresultant light is then combined with the original distribution of lightsplit beam splitter 108 and is deterred by mirror reflector 110 andmirror reflector 112, thereby the resulting aerial image is inclusive ofonly the principle harmonic of the image. The desired aerial image isthen made incident on the resist layer 126 coating the wafer 128. Theimage exposed onto the resist layer at this point has an enhancedcontrast, and therefore the image also exhibits improved resolution.

Referring now to FIG. 2, a diagram detailing the effects of the presentinvention on the different orders of light as it passes through a maskis depicted in accordance with a preferred embodiment of the presentinvention. Light 202 from the illumination source again passes throughmask 204 only at specific openings set within the opaque material inmask 204. Specifically, light passes through opening 206 in mask 204 andare blocked by opaque portions 208 and mask 204. FIG. 3 is a defractionpattern graph. When light 202 passes through opening 206 of mask 204,light 202 through diffraction, acquires additional and unwanted subharmonics as depicted in the diffraction pattern graph 300 in FIG. 3.

With reference now to FIG. 4, a relative intensity distribution graph isdepicted. The light pattern, after diffraction, is again represented inthe relative intensity distribution graph 400. This graph shows a firstorder of light 402, a second order of light 404, and a third order oflight 406 alone with a original zeroeth order of light 408. With theseundesired harmonics, the aerial image when made incident upon the resistlayer exhibits degradation in the resultant resist image, therebylimiting the proximity that two neighboring features can be placedtogether. When the sub harmonics are not countered, the diffractedaerial images from the two neighboring features will coincide, whichresults in a loss of resolution.

With reference now to FIG. 5, a relative intensity distribution graph500 depicting the result of filtering the light as it passes through thephase shifted intensity filter cell is illustrated in accordance with apreferred embodiment of the present invention. In relative intensitydistribution graph 500, the zeroeth order of light has been removed fromthe light, and the remaining sub harmonics have been phase shifted180-degrees. As can be seen, the first order of light 502, second orderof light 504, and third order of light 506 are phase shifted 180 degreesfrom those in FIG. 4. When the resulting light is then combined with theoriginal light pattern the 180-degree phase shifted harmonics and theiroriginal respective unfiltered and non-shifted harmonics cancel eachother out, leaving only the desired zeroeth order of light as exhibitedin the final intensity distribution graph 500.

With reference now to FIG. 6, a relative intensity distribution graph600 of light left from the filtered light is depicted in accordance withpreferred embodiment of the present invention. In relative intensitydistribution graph 600, a zeroeth order of light remains afterfiltering.

The method and apparatus so described can be built in various mannersand forms depending on the implementation. In the depicted examplematerial 116 and material 118 must be situated and confined within anenclosed cell that is encapsulated with an anti-reflective material, andthe exiting end is covered with a 180-degree phase shifting compound.

Thus with deep ultraviolet lithography and the constant demand forsmaller and closer set features, the present invention provides therequired resolution increases needed to maintain functionality of thecomponents with the semiconductor devices. Thus, the present inventionprovides an improved approach to the concept of contrast enhancement byintroducing an “intensity filtration” system between the resist and thelens, thereby preventing the sub harmonics from travelling to the resistlayer. The present invention provides this advantage by filtering offharmonics in the intensity distribution of the aerial image in a DUVlithography system. By filtering off specific harmonics, the method andapparatus provides an improved contrast in the resist image. Theresulting image will also be free of harmonic sidelobes. Theseimprovements allow two adjacent features to be located closer togetherwithout interference from each other. Without the enhancement incontrast the same features in the same locations would lose resolutiondue to the fact that the diffracted aerial images of the two featureswould coincide.

The description of the preferred embodiment of the present invention hasbeen presented for purposes of illustration and description, but is notlimited to be exhaustive or limited to the invention in the formdisclosed. Many modifications and variations will be apparent to thoseof ordinary skill in the art. The embodiment was chosen and described inorder to best explain the principles of the invention the practicalapplication to enable others of ordinary skill in the art to understandthe invention for various embodiments with various modifications as aresuited to the particular use contemplated.

1. An intensity filter for enhancing contrast of an image created bypassing light through a mask, wherein the described intensity filtercontains: a first material and a second material, wherein the firstmaterial and second material interface in such a way that only specificintensities are passed through, wherein the first material is nonlinearin nature and has a refractive index that changes at high intensitiesbut has a constant refractive index equivalent to the second materialand wherein the second material has a constant refractive indexregardless of varying levels of intensity, at intensities lower than aspecific minimum threshold, wherein contrast of the image is enhanced bythe filter, and a coating that phase shifts exiting filtered light. 2.The intensity filter of claim 1, wherein the coating phase shifts theexiting filtered light by 180 degrees.
 3. A method for forming aphotoresist material during fabrication of an integrated circuit, themethod comprising: providing a masking member; and providing a filterused to filter light having a range of intensity from a first intensityto a second intensity, wherein the filter includes a first material anda second material, the first material having a constant refractive indexand the second material having an intensity dependent refractive index,wherein at the first intensity the constant refractive index is aboutthe same as the intensity dependent refractive index and at the secondintensity the intensity dependent refractive index changes such that arefractive index mismatch occurs between the first material and thesecond material, projecting light though the masking member onto a layerof photosensitive material, wherein light of the second intensity isblocked by the filter.
 4. The method of claim 3, wherein the filterincludes a phase shifter, which produces a 180 degree phase shift inlight filtered by the filter.
 5. A filter used to filter light having arange of intensity from a first intensity to a second intensity, thefilter comprising: a first material and a second material, the firstmaterial having a constant refractive index and the second materialhaving an intensity dependent refractive index, wherein at the firstintensity the constant refractive index is about the same as theintensity dependent refractive index and at the second intensity theintensity dependent refractive index changes such that a refractiveindex mismatch occurs between the first material and the second materialand wherein light of the second intensity is blocked by the filter frombeing projected through the masking member onto a layer ofphotosensitive material, and a phase shifter that shifts a phase of thelight projected onto the layer of the photosensitive material.
 6. Thefilter of claim 5, wherein the first material has a refractive indexthat is selected as follows:n=n ₀ wherein n is a refractive index and n₀ is a constant.
 7. Thefilter of claim 6, wherein the second material has a refractive indexthat is selected as follows:n=n ₀ , E>Ean=n ₀ ±n ₀ *E/Ea, E<Ea wherein n is a refractive index and n₀ is aconstant, E is an intensity of incident light, and Ea is an intensity atwhich a material switches from a constant index of refraction to anintensity dependent refractive index of refraction.
 8. The filter ofclaim 6, wherein the second material isN-(p-methoxylbenzylidene)-p-butylaniine.
 9. The filter of claim 5,wherein the phase shifter shifts the phase of the light projected ontothe masking member by 180 degrees.